Pneumatic tires have sidewalls which are conventionally desired to have good flexibility yet be also scuff resistant.
Accordingly, scuff resistance is desirable for a tire sidewall. However, flexibility is also a conventionally required attribute for rubber tire sidewalls.
Historically, where a portion of a tire sidewall is of a color (e.g. white) in contrast with the remainder of tire, or sidewall (e.g. black), it is normally desired that the tire sidewall rubber composition be of a non-staining rubber composition to prevent, or at least retard, a staining of a tire sidewall having a color (e.g. white) other than the adjoining black color of the tire. This desirability is well known to those having skill in such art.
However, it is also well known that, in general, tire sidewall rubber compositions typically contain ingredients that are considered to be staining in nature for colored sidewalls such as, for example, amine-based antidegradants, unless a rubber composition adjoining the colored (e.g. white) rubber composition in the sidewall is especially compounded to not contain such staining ingredient (e.g. does not contain amine-based antidegradant).
Where it is desired that the sidewall rubber composition be non-staining in nature, or at least have a reduced tendency to stain, amine-based antidegradants are typically replaced with phenolic antioxidants in the rubber composition as is well known by those having skill in such art.
However, use of phenolic antioxidants for a rubber sidewall composition are usually not as desirable as amine-based antioxidants for tire sidewalls simply because they are not effective for protection against ozone attack and resultant degradation of various properties of sulfur-vulcanized rubber compositions of diene-based elastomers.
Historically, scuff resistance of a tire's rubber sidewall is conventionally enhanced, for example, by inclusion of cis 1,4-polybutadiene rubber in its rubber composition as is well known to those having skill in such art.
Historically, flexibility of a tire's rubber sidewall, while conventionally provided by an inclusion of cis 1,4-polyisoprene in blends with high cis 1,4-polybutadiene rubber in its rubber composition, particularly natural rubber, may sometimes also be enhanced by an inclusion of an aromatic rubber processing oil in its composition, as is well known by those having skill in such art.
It has been observed herein that the replacement of conventional rubber processing oils with selected hydroxyl terminated liquid polyalkylene-based polymers resulted in improved resistance to flex fatigue in the absence of amine-based antioxidants for a sulfur-cured tire sidewall rubber composition which was composed of cis 1,4-polybutadiene, a brominated copolymer of isobutylene and p-methylstyrene and a minor amount of cis 1,4-polyisoprene.
It has further been observed herein that a replacement of conventional rubber processing oil in a tire sidewall elastomer composition of natural rubber (cis 1,4-polyisoprene) and cis 1,4-polybutadiene with a staining antioxidant (an amine-based antidegradant) resulted in no improvement in tire sidewall rubber composition flex endurance related properties.
Therefore, a means of providing a tire with a suitably flexible sidewall yet maintaining resistance to ozone attack is desired in order to provide a tire sidewall of a conjugated diene rubber composition which also contains sufficient saturated, or essentially saturated with only minimal unsaturation, polymers so that it can be produced without a requirement for use of staining amine antidegradants.
Historically, it has sometimes been proposed to use various diene-based liquid polymers which contain carbon-to-carbon double bond unsaturation to replace at least a portion of rubber processing oil contained in various rubber compositions. One philosophy has been for the liquid polymer to initially enhance the processability of a high viscosity unvulcanized rubber composition by reducing its viscosity and to later co-vulcanize with the elastomer upon vulcanizing the rubber composition.
Such use of rubber processing oil and proposed use of unsaturated, diene-based liquid polymers is well known to those having skill in such art.
Indeed, while rubber processing oils have been used to improve the processability of various unvulcanized high viscosity elastomers, the inclusion of processing oils in such rubber compositions often results in a decrease in their vulcanized modulus of elasticity. Accordingly, the use of substantial amounts of processing oils in rubber compounds for a purpose of enhancing their unvulcanized processability is not necessarily a desirable option.
In the description of this invention, the term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber elastomer". The terms "rubber" and "elastomer" can be used interchangeably, unless otherwise distinguished. The terms "rubber composition", "compounded rubber" and "rubber compound" can be used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and the terms "cure" and "vulcanize" may also be used interchangeably herein, unless otherwise noted and such terms are well known to those having skill in the rubber mixing or rubber compounding art.